How to Encode an Image as a BMP File

The BMP format (“Bitmap Picture”) is one of the oldest and simplest formats for storing digital images. Originally designed for Windows, this format provides an uncompressed representation of images, although it also supports certain forms of compression. Below, we explore how an image is encoded in this format.

Basic Structure of a BMP File

A BMP file consists of several main sections that contain information about the image and its data. These sections are:

1. File Header

• This part occupies 14 bytes and contains general information about the file.
• Main fields:
  • Type (2 bytes): Identifies the file format. In BMP, this field contains the letters “BM”.
  • Size (4 bytes): Indicates the total file size in bytes.
  • Offset (4 bytes): Specifies the start of the image data within the file.

2. DIB Header

• Describes the image properties, such as dimensions, color depth, and compression.
• Variable size, depending on the BMP version.
• Common fields:
  • Header size (4 bytes): Specifies the length of this section.
  • Width and height (4 bytes each): Dimensions of the image in pixels.
  • Color planes (2 bytes): Always set to 1.
  • Bits per pixel (2 bytes): Defines the color depth (e.g., 1, 4, 8, 16, 24, or 32 bits).
  • Compression (4 bytes): Indicates if compression is used and what type (e.g., uncompressed, RLE).
  • Image data size (4 bytes): Size of the pixel array.

3. Color Palette (optional)

Used if the image has 1, 4, or 8 bits per pixel. Each entry represents a color and occupies 4 bytes (values for red, green, blue, and reserved).

4. Image Data

• Contains the pixel values that make up the image.
• The format depends on the color depth:
  • For 24 bits, each pixel has 3 bytes (blue, green, red).
  • For 8 bits, each pixel is an index in the color palette.
• Data is stored from bottom to top and left to right. This means the first pixel corresponds to the bottom-left corner of the image.

Step-by-Step Encoding

1. Prepare the image data: Convert the image to a compatible format (e.g., 24 bits). Determine the width, height, and color depth.
2. Create the file header: Specify the type (“BM”), the total file size, and the offset to the data.
3. Build the DIB header: Include information about dimensions, color planes, depth, and compression.
4. Write the color palette (if needed): Generate a color table for low-depth images.
5. Save the pixel data: Arrange the pixel data in the proper format and write it to the file.

Advantages and Disadvantages of the BMP Format

Advantages:

• Simplicity in implementation.
• High image quality, as it does not use lossy compression.

Disadvantages:

• Large file size due to lack of compression.
• Less support in modern applications compared to formats like PNG or JPEG.

The BMP format remains useful in scenarios where simplicity and quality are priorities, such as in certain technical and development applications. Understanding its structure helps programmers work efficiently with this type of file.